Calories in weight training

[Anoninone]

Hi. I have had my Fitbit charge 5 for 2 days now. I have compared it to my polar a370 on my weight training. Polar shows 500kcal and 400kcal on the same workouts that Fitbit shows 350kcal and 200kcal. How do I know who is most correct? I am 180cm and weight 59kg. 

[t.parker]

Simply, you can't know. Calories are just estimated and both devices use different algorithms. If you going for weight loss then pick lower number (it doesn't mean it's correct but puts you on the safe side), otherwise you need to figure out yourself. The device gives you some estimate but it's not something it is able to measure like for example HR.

[Heybales]

Did the Fitbit allow you to pick a workout for Weights that you started?

That is likely more accurate then.

Compared to Polar is likely just getting a HR reading and using their standard formula for calories - which is wrong usage for anything but steady-state cardio.

Lifting is not steady-state - your HR is all over the place up and down, like intervals. And if done right, it's also anaerobic.

Both lead to inflated calorie burn using calculations. Which is what Polar is probably doing. Their white-paper claims to try to tell the difference in your activity to not doing that - but there are complaints about it actually working.

But if this is a Fitbit model that uses the Weight lifting workout, it's using a rate of burn as found in the database, that database is from the METS database where rate of burn is based on studies at least.

For Weights that is with 2-5 sets, 5-15 reps, 2-5 min rests.

About 3.5 x your BMR/1440 per min. 

So if BMR is 1500 and you did 60 min - 3.5 x 1500 / 1440 x 60 = 219 cal.

For Circuit Training, 15 or more reps, up to 1 min rest, that's about 8.0 METS, closer to cardio.